thermal physics. temperature measures the tendency for energy to leave an object spontaneously a...

Thermal Physics

TEMPERATURE

•measures the tendency for energy to leave an object spontaneously•a measure of the average kinetic energy of the molecules in the object

Will both fingers feel the same temperature when they'reput in the warm water?

Thermal Equilibrium

•Two objects in contact are said to be in equilibrium if their temperatures are the same

Does the thermometer measure the temperature the water had before the thermometer was put in?

Zeroth law of equilibrium

If body A is in thermal equilibrium with body B and body B is in thermal equilibrium with body C, then body A is in thermal equilibrium with body C.

Kelvin and Celcius Temperature Scales

•Temperature is a direct measure the average translational KE of the molecules or atoms in the material•High temperature high average KE

The Kelvin scale directly measures the average KE of the molecules

Kelvin is identical to Celcius except the zero point is set at -273.15o CCC

Internal Energy, U

•The sum total of all the energy associated with molecular motions and positions

Work & Heat

•Work has been done ..if the energy exchange is associated with organized motion of molecules in the surroundings

Work & Heat

•Heat.. the transfer of energy is a result of random motions in the surroundings•Heat can be transferred only between systems that are at different temperatures

First Law of Thermodynamics

Heat Transfer Mechanisms

CONVECTION

Convection is the process in which heat is carried from one placeto another by the bulk movement of a fluid

convection currents

CONDUCTION

Conduction is the process whereby heat is transferred directly througha material

One mechanism for conduction occurs when the atoms or moleculesin a hotter part of the material vibrate or move with greater energy thanthose in a cooler part.

By means of collisions, the more energetic molecules pass on some oftheir energy to their less energetic neighbors.

CONDUCTION OF HEAT THROUGH A MATERIAL

The heat Q conducted during a time t through a bar of lengthL and cross-sectional area A is

L

tTkAQ

SI Units of Thermal Conductivity: J/(s·m·Co)

thermalconductivity

THERMAL RADIATION

Radiation is the process in whichenergy is transferred by means ofelectromagnetic wavesIt doesn’t need a material medium between the two systems exchanging heat.

IDEAL GAS

ASSUMPTIONS ABOUT THE GAS1. On average, the space between molecules is much larger than the actual size of the molecules2. Molecules collide elastically with the container walls and each other3. While molecules exert forces on each other during a collision, there are no long-range forces between them, so there is no internal potential energy

Boyle’s Law - Temperature and moles of gas are held constant

pV = p’V’

The relationship between volume and pressure in a contained gas is an indirect proportion

Kinetic Molecular Theory

A molecule of mass m collides elastically with one wall

∆p = -mv – mv = -2mv

a = L

∆t = 2L / v

fav = ∆p / ∆t = -2mv/ (2L/ v) = mv2/ L

P = fav / area = fav / L2

Assume 1/3 of all molecules are moving in this back-and -forth manner…

P = 1/3 N (mv2/ L3) = 1/3 N (mv2/ V)

PV = 1/3 N (mv2) = 2/3 N (1/2mv2) = 2/3N(KEav)

PV = nRT = NkT

k = R/6.02 x 1023 = 1.38 x 10-23 J/K (k = Botlzmann’s constant)

KE = 3/2 kT U = 3/2NkT = 3/2nRT